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A scheduled denial of service attack against Microsoft's main software update Web site did not materialize Saturday, as computers infected with the W32.Blaster worm failed to find their target.

Blaster first appeared on Monday and quickly spread to computers worldwide by exploiting a known security vulnerability in Microsoft's Windows operating system.

By Friday, the worm, which targets a Windows component for handling RPC (Remote Procedure Call) protocol traffic called the Distributed Component Object Model (DCOM) interface, spread to more than 423,000 systems, according to Oliver Friedrichs of Symantec.

In addition to infecting vulnerable Windows machines, Blaster worm was programmed to launch a denial of service (DOS) attack against windowsupdate.com, an Internet domain owned by Microsoft and used to distribute software updates to Windows customers beginning on Saturday.

However, an error in Blaster's design combined with last minute actions by Microsoft to change the registration of windowsupdate.com cut short that attack.

Blaster's author provided the incorrect domain address for windowsupdate. The address specified in the worm's code, windowsupdate.com, simply forwards users to the actual Windows update site, windowsupdate.microsoft.com, according to Mikko Hyppönen, head of antivirus research at F-Secure Corp. in Helsinki.

On Thursday, Microsoft delisted the windowsupdate.com domain name, calling it a "nonessential address."

That solution also removed the threat of collateral damage from the attack, because requests for windowsupdate.com would never leave infected machines, slowing down the Internet, according to Sean Sundwall, a Microsoft spokesman.

The SANS Institute's Internet Storm Center reported Saturday that the DOS attack anticipated from Blaster had been avoided.

The number of Blaster infections is also down more than 80 percent since the worm's peak on Monday, indicating that vulnerable computers are being cleaned and patched by their owners, Friedrichs said.

Symantec expects that trend to continue, he said.

In time, Blaster will join predecessors like the Code Red and Nimda worms, inhabiting a small population of infected machines that pose a risk to new, unpatched systems, but not spreading much beyond that, he said.